Cameras improved as well. The long lens of the British Type L enabled an observer to fly higher, out of the reach of “Archie” (anti-aircraft fire). Still, it was one thing to take a good aerial photograph and survive to bring it home, and quite another to train someone who had never seen the world from the air to make sense of it.

A photograph is a near-exact projection of the terrain upon a two-dimensional plane, and in the words of Notes, people without “air-sense” literally could not tell a haystack from a hole in the ground. France led the pack in photographic analysis. Capitaine Jean de Bissy published his Note Concernant l’Interpretation Methodique de Photographies Aeriennes a year before the war commenced. De Bissy’s pamphlet became the model for all subsequent aerial photographic training. It morphed into a number of longer, more detailed publications that, unlike the high-quality camera lenses they hoarded, the French willingly shared with their British allies.

Lieutenant J.T.C. Moore-Brabazon, who in 1909 had won London’s Daily Mail prize for the first all-British flight of a circular mile, tried to get someone interested in the French manual. He had a hard time. The new technology seemed rude. “The Army took the greatest exception to an enemy who indulged in dirty tricks,” he wrote in his 1956 memoir, The Brabazon Story. “Aerial photography invaded a privacy that had always been accorded an enemy.”

The war quickly rendered peacetime courtesies obsolete, and the British army published Notes on the Interpretation of Aeroplane Photographs, the English version of the original French book, in November 1916. When America entered the war the next year, Major James Barnes and First Lieutenant Edward Steichen—the former an explorer, the latter the most famous photographer of his time—surveyed French and British material on aerial photography. Together, they produced the copy of Notes issued to my literary agent’s father back in 1918.

So just how much do the plates in Notes reveal to an infantry intelligence officer like Captain Burger back in 1918? Quite a bit.

Where was the enemy? Take a look at how markings left by recently buried communications cables point, unerringly, to what we today might call a “command and control node.” What were the enemy’s strengths? Look how much information the analyst squeezed from one plate: billets for troops, hidden heavy guns, batteries of howitzers, even trucks on the road. What is the enemy up to? We see “No-Man’s Land” being prepared for an all-out attack: new “saps” (exploratory trenches) snaking out from German-held territory to foot trails left by daring British patrols.

The publication of Notes marked the end of aviation and aerial photography as primitive arts, and their birth as sciences.

Not that art and inspiration were completely banished. In a postwar memoir, Notes co-author James Barnes writes: “Interpreting aerial photographs demands a peculiar mind—the type that enjoys chess problems or crossword puzzles. To the uninitiated, a photograph of a line of trenches and myriad shell holes means very little. But to a puzzle solver, they tell a story. Often his imagination is set on fire by some puzzling little thing, the reason for which he cannot quite discover. Like a game of poker with aces up the sleeve, the battle between the camera and camouflage is on. And then, all at once, he has it!”

Today, the puzzle solvers can be found at places like the National Reconnaissance Office in Chantilly, Virginia, working to decipher images returned by U.S. reconnaissance satellites that continuously orbit over the world’s trouble spots. Despite the many advances in technology, these analysts know exactly what Barnes was talking about 90 years ago: the thrill of having your imagination set on fire by some puzzling little thing, and then, all at once, you’ve got it. The enemy is revealed.